Urinary System Overview

Urinary System Components

The urinary system comprises two kidneys, two ureters, a bladder, and a urethra.
Kidneys are the primary organs responsible for blood filtration.
Ureters transport urine from kidneys.
Bladder stores and expels urine.
Urethra is responsible for urine excretion.

Kidney Location and Movement

Kidneys are reddish-brown, bean-shaped organs located on either side of the vertebral column (VC).
They are retroperitoneal, meaning they sit behind the parietal peritoneum.
The left kidney sits at vertebral levels T12, L1, L2, while the right kidney is at L1, L2, L3.
The right kidney is lower due to the presence of the right lobe of the liver.
The upper part is protected by the 11th and 12th ribs, and the right kidney is only protected by the 12th rib.
Kidneys can move vertically within a limited range (~3cm) due to their fascial coverings.

Fascial Coverings

Kidneys are covered by multiple layers of fascia:
- Pararenal Fat: Extends across the lumbar region, thicker posteriorly and laterally.
- Renal Fascia: A condensed connective tissue membrane surrounding the kidney, perirenal fat, and suprarenal gland.
  - Divided into anterior and posterior layers which fuse laterally.
  - Superiorly, the two layers enclose the kidney and suprarenal gland.
  - Inferiorly, the layers extend along the ureter, allowing for limited mobility.
- Perirenal Fat: Surrounds both the kidney and suprarenal gland, separating them.
- Fibrous Capsule: Dense irregular connective tissue forming the outermost layer of the kidney.

External Anatomy of Kidney

The kidney has a convex lateral border, a concave medial border with the renal hilum, and an upper pole, middle area, and lower pole.
The Renal Hilum: The area where structures enter and exit the kidney.
- The transpyloric plane passes through the left hilum at the level of L1.
- The hilum opens into the renal sinus.
- Structures within the hilum (from anterior to posterior): Renal vein, Renal artery, Ureter.
- The left renal vein is longer than the right.
- The right renal artery is longer than the left.

Kidney Relations

Right Kidney:

- **Superiorly:** Liver (right lobe) and suprarenal gland.
- **Anteriorly:** Right colic flexure, second part of duodenum, and right lobe of liver.
- **Posteriorly:** Diaphragm, psoas major, and quadratus lumborum.

Left Kidney:

- **Superiorly:** Spleen and suprarenal gland.
- **Anteriorly:** Pancreas and left colic flexure.
- **Posteriorly:** Diaphragm, psoas major, and quadratus lumborum.

Internal Anatomy of Kidney

The kidney is divided into an outer cortex and an inner medulla.
- The cortex has two zones: cortical zone and juxtamedullary zone.
- The medulla contains renal pyramids (approximately 8-18).
From the medulla, the kidney shows projections into the cortex:
- Renal columns: Extend from the cortex between the pyramids.
- Medullary rays: Striated projections from the bases of the renal pyramids.
- The kidney is protected by a dense irregular connective tissue capsule.

Nephron: The Functional Unit of the Kidney

Each kidney contains approximately 1 million nephrons, responsible for blood filtration and urine production.
A nephron consists of a renal corpuscle and a renal tubule:
- Renal Corpuscle: Located in the cortex.
  - Contains the Glomerulus: A capillary network.
  - And the Glomerular (Bowman's) Capsule: A double-walled cup containing a visceral and parietal wall.
- Renal Tubule: Consists of the following segments:
  - Proximal Convoluted Tubule (PCT): Located in the cortex.
  - Loop of Henle: Extends into the cortex and medulla, with descending and ascending limbs.
  - Distal Convoluted Tubule (DCT): Located in the cortex, drains into collecting duct.
  - Collecting Ducts: Larger tubules receiving DCTs from multiple nephrons. A group of nephrons draining into a collecting duct form a renal lobule.
  - Papillary Ducts: Large ducts formed by the union of collecting ducts, located at the apex of renal pyramids. They drain into minor calyces.
Types of Nephrons:
- Cortical Nephrons: (80-85%) with glomeruli in the outer cortex and short loops of Henle.
- Juxtamedullary Nephrons: With glomeruli deep in the cortex and long loops of Henle penetrating the medulla.

Draining System

Each pyramid has numerous collecting ducts that gather to form papillary ducts of Bellini (~20).
These ducts drain through the renal papilla, the apex of the pyramid.
Each pyramid drains into a minor calyx (~8-18).
Minor calyces join to form major calyces (~2-3).
Major calyces join to form the renal pelvis.
The renal pelvis drains into the ureter.

Blood Supply to the Kidney

Arterial Supply:
- Renal Arteries: Enter the kidney at the vertebral level between L1-L2.
  - The right renal artery passes behind the inferior vena cava.
  - At the hilum, they branch into anterior and posterior divisions, which give rise to segmental arteries.
- Segmental Arteries: Located in the renal sinus branches, including:
  - Superior (Apical) segmental artery 1, Anterosuperior segmental artery 2, Anteroinferior segmental artery 3, Inferior segmental artery 4, and Posterior segmental artery 5.
- Lobar Arteries: One artery for each pyramid.
- Interlobar Arteries: Travel through the renal lobes.
- Arcuate Arteries: Arch over the bases of the pyramids between medulla and cortex.
- Interlobular Arteries: Located between renal lobules.
Venous Drainage:
- Renal Veins: Located anterior to the arteries.
- Left Renal Vein: Longer than the right and crosses the midline, receiving the left gonadal vein, left suprarenal vein, and the left inferior phrenic vein.
- Renal Vein Entrapment Syndrome (Nutcracker Syndrome): Occurs when the left renal vein is compressed between the superior mesenteric artery and the abdominal aorta, potentially affecting blood flow to the left kidney.